地震勘探中的几种常见“多初至”现象解析

Resolution of common multiple first breaks in seismic exploration

"多初至"现象在不同的地震施工条件下具有不同的形成机制,解决方法也各不相同。在沼泽区激发时,淤泥层的上下震荡在单炮记录上形成若干组多次初至波。淤泥层较薄时采用单深井在淤泥层以下激发,淤泥层较厚时采用试验所得参数进行组合激发,能有效消除"多初至"现象;在水中激发时,为消除气泡效应导致的"多初至"现象,可选择激发深度小于初始气泡的半径;对于可控震源作业中因震源耦合影响造成的"多初至",应考虑降低驱动幅度或加载力控装置,也可适当加大扫描长度;此外,重视设备维护保养,可避免因设备原因导致的"多初至"现象。

In different exploration conditions,the cause of multiple first break phenomenon is different,so are the resolutions.When shooting in swamp areas,the silt seam will shake up and down,which generates several mutiple first breaks.In order to eliminate this phenomenon,if the silt seam is thin,shot must be conducted below the silt seam in single hole,and if not,shooting in array holes with parameters acquired in test. When shooting in water,to eliminate multiple first breaks caused by bubble effect ,the shooting depth must be less than the radius of initial air bubble.If source is vibrator and multiple first breaks are caused by coupling problem, it is effective to reduce force peak or furnish control device,or enlarge sweep length. The problem of multiple first breaks can be also avoided by keeping the equipments in good conditions.